The Basic Header is given in Block 1 of a FIN message and is the only header that appears on all messages. The Basic Header provides the fundamental reference for any particular message and is almost always automatically built by the FIN interface.

The Basic Header has the same format for both input and output messages. However, the information contained in the Basic Header is relative to the sender when the message is input but relative to the receiver when the same message is output.

The following is an example of a basic input header, as it might appear at the beginning of a user-to-user message input within FIN:

{1:F01BANKBEBBAXXX2222123456}

The components can be separated as follows

(a) Block Identifier
The Block Identifier for a Basic Header Block is always '1:'.

(b) Application Identifier
The Application Identifier identifies the application within which the message is being sent or received. The available options are:
F = FIN All user-to-user, FIN system and FIN service messages
A = GPA Most GPA system and service messages
L = GPA Certain GPA service messages, for example, LOGIN, LAKs, ABORT
These values are automatically assigned by the SWIFT system and the user's CBT, but the user should be aware of their existence and significance.

(c) Service Identifier
The Service Identifier consists of 2 numeric characters. It identifies the type of data that is being sent or received and, in doing so, whether the message which follows is one of the following:
a user-to-user message
a system message
a service message, for example, a session control command, such as SELECT, or a logical acknowledgement, such as ACK/SAK/UAK

In general you will be primarily concerned with identifier '01' which applies to all GPA and FIN system and user-to-user messages. Other values include: '21' for message acknowledgements such as ACK/NAK, UAK/UNK, '03' for SELECT commands, and so on.

(d) LT Identifier
This 12-character FIN address, given in the Basic Header Block, is the address of the sending LT for input messages or of the receiving LT for output messages, and includes the Branch Code.
In the Basic Header, the LT Code within the LT Identifier is specific to the LT that has established the session in which the message is being transmitted, that is the sending LT for input messages or the receiving LT for output messages.

(e) Session Number
The Session Number identifies the session in which the message was transmitted. Within the Basic Header, the 4-digit Session Number is the user's current GPA or FIN Session Number.

(f) Sequence Number (ISN or OSN)
The sequence number always consists of 6 digits. It is the ISN of the sender's current input session or the OSN of the receiver's current output session.
Required only in the following instances:

For all GPA messages/service messages with the following Service Identifiers, the Sequence Number must be equal to the next expected number:
o 01 (user-to-user messages)
o 03 (SELECT)
o 06 (LOGOUT)

For all GPA messages with the following Service Identifiers, the Sequence Number must be equal to that of the acknowledged service message:
o 21 (ACK/NAK/UAK/UNK)
o 23 (SELECT ACK (SAK))
o 26 (LOGOUT ACK)
o 43 (SELECT NAK (SNK))

For all FIN messages/service messages with the following Service Identifiers, the Sequence Number must be equal to the next expected number:
o 01 (user-to-user messages)
o 05 (QUIT)

For all FIN messages with the following Service Identifiers, the Sequence Number must be equal to that of the acknowledged service message:
o 21 (ACK/NAK/UAK/UNK)
o 25 (QUIT ACK)